During VOA Radiogram for the weekend on 30 November and 1 December, we noticed horizontal interference lines in the MFSK64 image of VOA correspondent Steve Herman, W7VOA. The transmission of this image was centered on 1000 Hz, thus occupying 500 to 1500 Hz. The second audio harmonic overlapped at 1000 to 3000 Hz. Did the overlap cause the lines?

During the past weekend, 7-8 December 2013, we transmitted the same MFSK64 image of Steve, first centered on 1500 Hz. In this case the fundamental was 1000 to 2000 Hz, and the second audio harmonic was 2000 to 4000 Hz. The we transmitted the same image centered on 1000 Hz, thus including the overlap.

The photos in the left column below were centered on 1500 Hz, in the right column on 1000 Hz. They are typical of several screenshots sent in by listeners. The overlap of the second harmonic, or some other phenomenon related to the center audio frequency, caused the horizontal lines.

During VOA Radiogram, 30 Nov/1 Dec, the 1000 Hz center audio frequency, with overlapping second harmonic, had no negative effect on the decoding of MFSK64 text.  

Centered on 1500 Hz                                    Centered on 1000 Hz

Netherlands 17860 kHz

Norway 17860 kHz

Greece 17860 kHz

UK 17860 kHz

Texas 5745 kHz